Conventionally known hydraulic drive systems of the above-described type are described in, for example, JP, B, 47-3927 and JP, B, 50-5354. Each of these known hydraulic drive systems comprises a variable displacement hydraulic pump, at least one actuator driven by a hydraulic fluid delivered from the hydraulic pump, a directional control valve of center bypass type having a meter-in passage provided with a meter-in variable restrictor and a center bypass passage provided with a bleed-off variable restrictor for controlling a flow of the hydraulic fluid supplied from the hydraulic pump to the actuator, a low pressure circuit, a center bypass line for connecting the center bypass passage to the low pressure circuit at a point downstream of the bleed-off variable restrictor, a pressure generator, e.g., a fixed restrictor, disposed in the center bypass line, and a pump regulator for controlling a displacement volume of the hydraulic pump using the pressure generated by the fixed restrictor as a control pressure.
The pump regulator performs well-known negative control based on the control pressure generated by the fixed restrictor. More specifically, the pump regulator controls a displacement volume of the hydraulic pump so that the displacement volume increases as the control pressure is lowered, and it decreases as the control pressure is raised.
In the prior art arranged as above, when the directional control valve is gradually shifted from a neutral position through its stroke with the intention of driving the actuator, an opening area of the bleed-off variable restrictor of the directional control valve is gradually reduced and, to the contrary, an opening area of the meter-in variable restrictor thereof is gradually increased.
When the directional control valve is in the neutral position or at the start point of the stroke thereof, i.e., when the bleed-off variable restrictor begins to close, the control pressure generated by the fixed restrictor is high and the hydraulic pump is kept at a predetermined small displacement volume to deliver the hydraulic fluid at a standby flow rate which is small corresponding to the predetermined small displacement volume. Then, as the bleed-off variable restrictor is gradually closed, a pressure of the hydraulic fluid delivered from the hydraulic pump, i.e., a pump pressure, is raised. Assuming now that a load pressure of the actuator is Pa, the actuator starts moving at the time the pump pressure is raised in excess of Pa. As the actuator starts moving and the hydraulic pump starts supplying the hydraulic fluid to the actuator, a flow rate of the hydraulic fluid passing through the center bypass passage is reduced accordingly. Such a reduction in the flow rate passing through the center bypass passage lowers the control pressure generated by the fixed restrictor in the center bypass line. Correspondingly, the pump regulator is driven so as to increase the displacement volume of the hydraulic pump. As a result, a delivery rate of the hydraulic pump is gradually increased to provide a predetermined flow rate characteristic, i.e., metering characteristic.